In order to deaden the fire in a stove, the dampers are closed to prevent air from entering; the fire is kept alight by the very small quantity of air that leaks in below the fire-box. When the drafts are opened the fire will burn up brightly because a plentiful volume of air can then enter. In a similar way, air must be used in a gas engine in order that the fuel may burn. It is not possible to mix air with a liquid; the first step in making a gas that will burn is, therefore, to turn the fuel, whether it is gasoline, kerosene, distillate, or other oil, into a vapor; this vapor is then mixed with air. For good results it is very necessary that the vapor and air be in proper proportions. In the experiment with the candle and the bottle it was seen that as the air was used up, the candle flame became yellow and smoky: this is the effect of insufficient air. If there is not enough air in the mixture, part of the vapor will not be able to burn, and will only smoke. If, on the other hand, there is too much air, the mixture, if it will burn at all, will burn slowly, and the extra volume of air will reduce the heat. In a mixture of the proper proportions of air and fuel vapor, the burning, or _combustion_, will be very rapid, resulting in the sudden production of the greatest possible amount of heat. This, of course, is what is necessary if the engine is to produce its fullest power. With such a mixture, combustion will be complete before the piston has done more than start outward on the power stroke, and the greatest possible, or _maximum_, pressure will then be produced. When a mixture burns slowly, the piston will have gone through much of the power stroke before combustion is complete, in which case a considerable part of the pressure that should have been applied at the beginning of the stroke will be wasted. A mixture that is not correct will burn unevenly; it may burn better during one power stroke than during another, which will make the engine run unsteadily. If the mixture has too much air in proportion to the amount of vapor, it is known as a _thin_ mixture, or a _lean_ or _poor_ mixture. It burns so slowly that it is quite possible for the mixture that started burning before the beginning of the power stroke to continue burning through the exhaust stroke, and for enough flame to remain in the cylinder to set fire to the fresh charge that enters during the next inlet stroke. This will produce what is known as a _backfire_; that is, the mixture entering the cylinder will catch fire, and in burning will blow back through the open inlet valve. This is a dangerous condition, for the flame might spread to fuel dripping from the carburetor, or to the fuel tank. A mixture that has not enough air is called a _rich_ mixture; the air that is present will burn part of the vapor, while the rest will go out of the exhaust unburned, or will work past the piston into the oil in the crank case. This is wasteful of fuel.